The present invention relates to a device for pressure measurement on a line of various installations. It relates more particularly to a manometer whose design makes it possible, on the one hand, to avoid any dead space in the driving element, and on the other hand to offer very easy cleaning without the need to disconnect the measuring instrument from the process line or even dismantle it.
Manometers of conventional design, whether of the Bourdon tube, capsule, or bellows type, etc., have driving elements containing a relatively large dead space, in which the fluid to be measured remains trapped.
In manometers of this type, it is in fact very difficult, if not impossible, to clean the said driving element, which is at a real dead end that is impossible to reach. Finally, the fluid whose pressure is being measured remains permanently trapped inside the driving element. For certain industries and applications, this phenomenon may be troublesome, because undesirable germs and bacteria can grow and multiply inside this dead end. Moreover, it is obvious that by virtue of its very shape, a Bourdon tube or bellowsxe2x80x94whether it is drawn without welding, or rolled and welded then drawnxe2x80x94cannot have a perfect internal surface condition whatever treatments are applied to it.
Assuming that the internal surface condition is correct, the welds producedxe2x80x94at the end of the tube and for fixing the tube to the connectorxe2x80x94give rise to the presence of impurities inside the tube.
Treatments such as electropolishing, treatment in the xe2x80x9cextrudomxe2x80x9d or other treatments, may claim to improve the internal surface condition of a really small part of the Bourdon tube, but in no case can they make it sufficiently clean and free from all impurities.
Conventional driving elements, of whatever form (Bourdon tube, bellows, capsule, etc.), prove to be real pockets for particles, which are impossible to clean efficiently, and whose internal surface condition in contact with the medium to be measured is incompatible with industries requiring high degrees of cleanliness and purity.
Furthermore, during changes in manufacture, the user may wish to clean the lines and the measuring instruments that are installed on the latter. In the case of manometers, such cleaning is impossible, unless the manometer is separated from the installation by means of an intermediate component, called a separator. This separator makes it possible to isolate the measuring instrument from the line, and serves as a xe2x80x9cbufferxe2x80x9d. The separator is equipped with a diaphragm that is in contact with the fluid to be measured, whilst a filling liquid (generally an oil) provides transmission of the pressure existing in the installation between this diaphragm and the driving element of the manometer.
The use of a separator may give rise to problems and is not always ideal. The performance of a separator largely depends on the mechanical characteristics of the diaphragm itself (its response curve), on the quality of the filling liquid, its thermal stability, its viscosity, the filling conditions, etc. In addition, hermeticity between this diaphragm and the line of the installation is provided by a seal, which also offers retention zones where germs, bacteria and microbial flora can develop.
Furthermore, the use of a filling liquid in the separator means there is a risk of contaminating the entire installation equipped in this way, if diaphragm rupture occurs.
In addition, a separator is an added item, independent of the manometer, which adds an extra cost to the final product.
Even so, manometers, whether or not they are mounted on a separator, are reliable instruments for measuring pressure, whose performance meets the requirements of the majority of industrial applications.
There are fields of application or industries where the drawbacks mentioned above become preponderant. This applies in particular to the food and agricultural industries, fine chemicals, the pharmaceutical industry, the semiconductor manufacturing industry, industries producing or using pure, rare and toxic gases, industries where pressure measuring instruments are used in painting processes, etc.
In all these industries or applications, the use of pressure measuring instruments demands very stringent precautions, especially with regard to the presence of impurities, germs, dust etc. The measuring instruments used in these types of processes must be able to be cleaned very easily.
In food and agricultural industries, for example, the processors need to measure the pressure of foodstuff liquid or pastes intended for human or animal consumption. Consequently, equipment for carrying out these measurements must never under any circumstances allow the growth and development of germs or bacteria that might alter or contaminate the foodstuffs whose pressure is being measured. Therefore the measuring instruments must be designed in such a way that the retention zones are almost non-existent, and in such a way that they are easy to clean (i.e. they must be designed in such a way that they can easily be cleaned by passing cleaning products, hot water, or other decontaminating products through the lines of the installation).
Similar problems arise in the pharmaceutical industry, fine chemicals industry, etc.
In the field of gas distribution in the semiconductor industry, manometers are mainly used for measuring the pressure of two groups of gases:
Gases that are called xe2x80x9cpure gasesxe2x80x9d, which have extremely exacting requirements in terms of purity: ultrapure nitrogen; argon; helium; etc. These are generally gases for which the degrees of purity may reach or exceed 99.99999%.
Gases called xe2x80x9cdoping gasesxe2x80x9dxe2x80x94generally highly toxic gases (arsine, boronxe2x80x94gallium, etc.)xe2x80x94for doping the silicon wafers on which electronic components are produced, such as memories (RAM, DRAM), microprocessors, etc.
In this industry it is necessary to employ measuring instruments that have been made following very rigorous procedures in terms of cleanliness, so that there is no risk of contaminating the gases that are used.
Furthermore, all retention zones are forbidden, because, in this case too, they promote the development of undesirable germs or bacteria.
Moreover, for example in pressure measurement on lines for painting processes, depending on the operations being carried out, the pressure measuring instruments have to measure the pressure of paints of different colours. Between two manufacturing operations, the manometers must be easy to clean and must not have any retention zones where paint from the previous manufacturing operation might contaminate the next manufacturing operation.
The examples given above are not limiting. There are many other manufacturing processes where the measuring instruments installed on the production lines must be as easy to clean as possible, with a design that does not permit any retention zones promoting the development of germs, bacteria and other substances that could contaminate the production process or the constituents that are involved in the said production process, such as, in particular, liquids, gases, etc.
It is clear from the foregoing that these instruments must, as far as possible, be able to be cleaned or rinsed. In this context, it can be seen that conventional manometers do not offer this facility by any means, and that their use poses enormous problems, unless they are combined with separators.
To overcome these drawbacks, the main manufacturers of pressure measuring instruments have in recent years developed devices called xe2x80x9cfull-bore pressure transmittersxe2x80x9d which avoid these contamination problems. In fact, various technologies derived from recently developed technologies in electronics have made it possible to devise pressure measuring instruments in which the driving element (the element that makes it possible to transform a physical quantityxe2x80x94pressurexe2x80x94into an electrical signal) is quite simply a tube, and the xe2x80x9cfluidxe2x80x9d whose pressure is to be measured circulates inside this tube. There are, for example, pressure transmitters that are completely free from retention zones and that offer maximum ease of cleaning. This equipment consists of a tube, on which a flat surface is machined, the dimensions and machining tolerances of which are known precisely. Film-screen strain gauges, arranged as a Wheatstone bridge (a network of resistances), are carefully positioned and glued on this flat surface. When this network of resistances is supplied with an external voltage, the Wheatstone bridge supplies an electrical signal whose value varies as a function of the deformation of the strain gauges when a pressure is exerted on the inside wall of the tube (at the level of the flat surface).
The output signal of the Wheatstone bridge therefore depends on the value of the resistances glued to this flat surface. The flat surface tends to be distorted under the action of the pressure. As it deforms, the flat surface also causes the strain gauges to undergo deformations, which are finally translated into a change in the value of each of the resistances or strain gauges. An immediate consequence is a change and/or deviation of the output signal at the terminals of the Wheatstone bridge under the action of the pressure. On the whole, the output signal is proportional to the deformation of the flat surface and hence to the pressure exerted by the fluid circulating in the tube on the zone of deformation of the said flat surface. The output signal of the Wheatstone bridge is then processed by an electronic system that generally supplies a current between 4 and 20 mA, corresponding respectively to zero pressure and to the maximum pressure for which the measuring instrument has been calibrated.
Transmitters of this type meet in full the requirements of cleanliness, purity and cleanability. But their use is subject to other constraints, the most important of which, for the user, is the need to provide an electrical supply to the Wheatstone bridge and hence to this type of pressure transmitter. The end user therefore has to make provisions for the laying of power cables on the installation and process for this equipment. In the semiconductor industry, for example, it is not uncommon to install several hundred pressure measuring instruments for monitoring the distribution of gases in the manufacturing process. Plant of this type can be heavy and expensive. Furthermore, only transmitters of this type must be combined with a unit for visualizing the measurement transmitted by the instrument: a display will permit visualization of the line pressure in real time.
Finally, a transmitter is usually more expensive than a manometer.
The present invention therefore aims to overcome these drawbacks, by proposing a device that is independent of any driving element and is autonomous in terms of energy source, and can largely eliminate problems connected with the purity, cleanliness and cleanability of the driving element.
For this purpose, the direct-passage manometer of the invention is characterized in that it has a body that is provided at its center with a hollowed curved surface, into which at least two transverse holes open, to allow the entry and exit of the fluid whose pressure is to be measured, this curved surface being sealed off by a diaphragm that is held in position on its seating by a load cell which interacts with the body via fixing means and orifices, the load cell being provided in addition with second fixing means for mounting a movement amplifying device resting on the diaphragm and connected to an indicator.